Zebra mussels are rapidly colonizing the Great Lakes and are spreading throughout the fresh waters of North America. The rate of colonization is dependent upon the reproduction of zebra mussel veligers. Veligers are the microscopic, free-swimming larvae of adult zebra mussels produced by the eggs of female zebra mussels which are fertilized outside the shell and hatch.
Since veligers are capable of actively swimming for 1 to 2 weeks, the population of the veligers can be dispersed over considerable distances from parent colonies. Within 3 weeks of hatching, the veligers reach the beginning of the settling stage. At this stage the veligers are referred to as "post-veligers". During this time, the post-veliger has developed a foot which allows crawling to aid the post-veliger as it searches for a hard substrate such as rock, metal, wood, plastic, vinyl, glass, rubber, or in some cases plant life, etc. to which they can attach. After attachment, the post-veligers continue metamorphosis and are transformed into a shelled adult zebra mussel having a size typically ranging from 1.0 to 1.5 inches in length.
The adult zebra mussel may remain attached to the substrate throughout their life by byssal threads produced by a gland in the mussel. Depending upon the environment, zebra mussels have an average life span of 3.5 years and in some cases 5 years. Overpopulation of zebra mussels causes severe macrofouling of water supplies and reduction of plankton which other aquatic life uses for food. Another major concern is the clogging of pipes or process equipment, which can lead to efficiency loss, in industrial plants which can occur if zebra mussels attach to the inside of intake pipes, heat exchangers, screens, and other components through which water flows.
It is known to kill adult zebra mussels with chlorine dioxide by a continuous process. "Continuous" means the chlorine dioxide is fed into the aqueous system twenty-four hours daily, or substantially for 24 hours daily. In the continuous process, typically a free residue of from 0.1 ppm to 0.5 ppm of chlorine dioxide is continuously fed into a population of adult zebra mussel. In this continuous process, all of the water entering the system is treated with chlorine dioxide.